Francis,
Since no one has an answer here, I suggest taking it to the xen-users mailing list. That would also be the right place to ask if this might be a bug. Some developers hang out there.
To access xen-users, read:
http://xenproject.org/help/mailing-list.html
Russ...

For all issues involving compiling from source, please take your question to the xen-users mailing list. The people on that list include developers who can assist you with matters like this. We don't have many people with that skill set who follow the Q&A system.
To access xen-users, please see:
http://xenproject.org/help/mailing-li...

If you don't get an answer here, try xen-users:
http://xenproject.org/help/mailing-list.html
Some developers hang out there, along with some power users. You will probably get better results there than here, which focuses on very simple issues.
Russ...

For the particulars of performance tuning, I recommend asking this question in the xen-users mailing list. You will have access to many more experienced users and a number of developers there.
To access xen-users, read the following:
http://www.xenproject.org/help/mailing-list.html
Russ...